This invention relates generally to a process for improving the shelf life of hindered phenol antioxidants by intimately contacting it with an sulfur-containing peroxide decomposer.
Polymers are subject to degradation by environmental forces, such as actinic radiation, oxidation, moisture, atmospheric pollutants and combinations thereof. Degradation, which primarily consists of a change in molecular weight of the polymers, may result in discoloration, brittleness, loss of clarity and mechanical strength, surface crazing and other manifestations.
Antioxidants are often used during the processing of polymers, such as polymer extrusion and molding, to inhibit or retard polymer oxidation and it ensuing degradative effects.
Oxidative degradation of polymers is a sequential process involving initiation, propagation, and termination phases. The initiation phase is started by the formation of free radicals, which may be produced by a number of factors such as the presence of reactive peroxides in the polymerization step, thermal, mechanical and radiation stresses during processing or end-use, or chemical reactions with impurities in the polymer. During the propagation phase, these radicals react with oxygen to form peroxy (ROO.) and alkoxy (RO.) radicals which in turn abstract hydrogen from the polymer to form unstable hydroperoxides (ROOH), alcohols (ROH) and new hydrocarbon free radicals (R.). These free radicals can once again combine with oxygen to continue the oxidative cycle.
Antioxidants can stop this oxidation cycle by interfering with the initiation and propagation steps. Primary antioxidants, such as hindered phenolics and secondary amines, are radical scavengers and react with free, peroxy and alkoxy radicals. Secondary antioxidants, such as phosphites and thioesters, act as peroxide decomposers and react with the unstable peroxides (ROOH) to form more stable alcohols.
One of the problems with some of the hindered phenol primary antioxidants is that they do not have long shelf lives. They tend to yellow with age, which is undesirable because the yellowness imparts color to the polymer. The present invention relates to a process of improving the shelf life of a hindered phenol antioxidant by intimately mixing it with a sulfur-containing peroxide decomposer thereby reducing the yellowness that results from age.
A number of publications, such as U.S. Pat. Nos. 4,820,755; 5,155,153 and 4,579,900, have disclosed the combination of hindered phenol antioxidant and thioesters. However, these patents only disclose the use of these components to stabilize a polymeric composition, and are only mixed together at the time of processing the polymer. There is no disclosure or teaching in these documents on increasing the shelf life of a hindered phenol antioxidant by mixing with a sulfur-containing peroxide decomposer.
The present invention relates to a process for improving the shelf life of a hindered phenol antioxidant comprising the step of intimately mixing the hindered phenol antioxidant with a sulfur-containing peroxide decomposer. The inventors have discovered that mixing the peroxide decomposer with the hindered phenol antioxidant reduces the tendency of hindered phenols to yellow with age. This increases the desirability of the hindered phenol because it will not impart color to polymer systems.
The present invention also relates to a composition produced from the process described above, and stabilized compositions and additive packages containing the composition produced from the above-described process.